Method and apparatus for reading invisible symbol

ABSTRACT

An invisible symbol reading apparatus includes a heating unit for heating an invisible symbol formed on a sample and containing a material which emits infrared light when heated, a detecting unit for detecting infrared light emitted from the invisible symbol, and an arithmetic operation unit for binarizing a detection signal from the detecting unit. The arithmetic operation unit calculates a differential coefficient of the detection signal, that corresponds to a position on the sample. On the basis of upper and lower threshold values set for the differential coefficient, the arithmetic operation unit determines a maximum value of the differential coefficient in a region exceeding the upper threshold value and a minimum value of the differential coefficient in a region smaller than the lower threshold value. The arithmetic operation unit binarizes the detection signal by using the maximum or minimum value as a leading or trailing edge of a binary function.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method and apparatus forreading invisible symbols (linear barcodes and two-dimensional symbols)usable in mail service, distribution of articles whose appearances areimportant, management of documents and articles requiring secrecy, andprevention of forgery.

[0002] Barcodes or two-dimensional symbols representing informationabout a manufacturing country, manufacturer, and product items areprinted or pasted as labels on many articles currently distributed onthe market. When an operator uses an optical reader to read a barcode, acorresponding price is read out from a database. This greatly reducesthe working time compared to conventional work in which an operatorinputs a price into a register. Stock control is also made efficient byconstructing a POS system. Additionally, barcodes are effective toincrease the efficiency of express delivery.

[0003] On the other hand, barcodes cannot be attached to some articles,and it is better not to attach barcodes to some articles presentlyhaving barcodes. For example, a printed barcode spoils the appearance ofa book. This problem of appearance can be solved if an invisible orstealth barcode can be attached to an article. A tag or barcode ispresently attached to the inside of a linen supply or clothing itemwhere this tag or barcode is difficult to see. However, the work ofdistribution can be rationalized if an invisible barcode can be attachedto the front surface of a packaged article.

[0004] In mail service, zip codes are read by an OCR to process a largeamount of mails within a short time. However, this zip code reading istime-consuming and requires manual sorting of mails because read errorssometimes occur. Although barcodes may be used in mail service, visiblebarcodes cannot be printed on the. surfaces of mails because thebarcodes contaminate the mails. If information such as a zip code can beprinted as an invisible barcode as in the above case, the time ofsorting can be greatly reduced, and this allows rapid delivery of mails.

[0005] A barcode can contain much information in a narrow space.However, a barcode itself cannot unlimitedly shrink, so a fixedexclusive area is necessary. This exclusive area is not negligible ifthe size of an article is small. However, an invisible barcode can besuperposed on some other printed information and hence does not requireany exclusive area.

[0006] As a method of preventing forgery, invisible barcodes can becombined with another forgery preventing method. This may improve theeffect of preventing forgery. As described above, invisible barcodes canextend the range of application of barcodes.

[0007] Two kinds of invisible barcode methods are presently possible: inone method a barcode is read by ultraviolet light, and in the othermethod a barcode is read by infrared light. In the method usingultraviolet light, a barcode is formed by using a fluorescent dye whichdoes not absorb visible light. This fluorescent dye is excited byultraviolet light, and fluorescence whose wavelength is different fromthat of the excitation light is detected. In the method using infraredlight, a barcode is formed by using a metal complex which does notabsorb visible light. This metal complex is excited by infrared light,and fluorescence whose wavelength is different from that of theexcitation light is detected (“Stealth Barcodes”, Tsunemi Ooiwa, OplusE,No. 213, p. 83, 1997).

[0008] Unfortunately, the ultraviolet light method has the followingproblem. That is, fluorescent dyes are often added to paper and clothfor bleaching purposes, and these fluorescent dyes also emitfluorescence. Since interaction with ultraviolet light is transitionbetween electronic states of molecules, fluorescence less depends uponthe intrinsic nature of a substance. Therefore, it is highly likely thatreading of fluorescence emitted from an invisible barcode is interferedwith. Also, a fluorescent dye in the ultraviolet region readily causesphoto-deterioration, so it is highly possible that no predeterminedfluorescence intensity can be obtained after a long-time use or storage.For these reasons, the reading accuracy largely declines easily.

[0009] Additionally, both fluorescent dyes and metal complexes haveproblems in toxicity and waste disposal. That is, barcodes are broughtinto homes together with commodities, and some barcodes remain existingin the living environment for long time periods. Therefore, babies andlittle children may lick these barcodes by mistake, or toxiclow-concentration exposure to barcodes may occur. When thesepossibilities are taken into consideration, it is necessary to selectmaterials from compounds already found to be safe. Furthermore, whenrecent waste disposal regulations are taken into consideration, it isdesirable to select materials by taking account of even recycling andfinal disposal. In these respects, it is preferable to avoid the use offluorescent dyes and metal complexes.

[0010] To prevent forgery, it is important that both a reader and aninvisible barcode material be difficult to obtain. When an ultravioletfluorescent dye is used, a light source for emitting ultraviolet lightis readily available. Fluorescence in the visible light region can, ofcourse, be visually checked. Fluorescence in the ultraviolet region isalso easy to visually check by inputting the fluorescence into anothermaterial. Additionally, fluorescence less depends upon the intrinsicnature of a substance, so a substance which emits fluorescence similarto that of a visible barcode material is readily obtainable. On theother hand, in the infrared light method using a metal complex, an LEDfor the near infrared region can be used as a light source, andfluorescence can be detected by a CCD camera. Additionally, a similarfluorescent material can be easily obtained as in the case of afluorescent material in the ultraviolet region.

[0011] As described above, an invisible barcode presently has manyproblems although it is expected as a technology meeting various needs.

BRIEF SUMMARY OF THE INVENTION

[0012] It is an object of the present invention to provide a method andapparatus capable of reading invisible symbols with high readingaccuracy.

[0013] A method for reading an invisible symbol of the present inventioncomprises the steps of heating an invisible symbol formed on a sampleand containing a material which emits infrared light when heated,detecting infrared light emitted from the invisible symbol, calculatinga differential coefficient of a detection signal corresponding to aposition on the sample, determining, on the basis of upper and lowerthreshold values set for the differential coefficient, a maximum valueof the differential coefficient in a region exceeding the upperthreshold value and a minimum value of the differential coefficient in aregion smaller than the lower threshold value, and binarizing thedetection signal by using the maximum or minimum value as a leading ortrailing edge of a binary function.

[0014] In the method of the present invention, it is also possible toheat the sample and detect infrared light emitted from the invisiblesymbol in a process of cooling the sample.

[0015] An apparatus for reading an invisible symbol of the presentinvention comprises heating means for heating an invisible symbol formedon a sample and containing a material which emits infrared light whenheated, detecting means for detecting infrared light emitted from theinvisible symbol, and an arithmetic operation unit for binarizing adetection signal from the detecting means. The arithmetic operation unitcalculates a differential coefficient of a detection signalcorresponding to a position on the sample, determines, on the basis ofupper and lower threshold values set for the differential coefficient, amaximum value of the differential coefficient in a region exceeding theupper threshold value and a minimum value of the differentialcoefficient in a region smaller than the lower threshold value, andbinarizes the maximum or minimum value as a leading or trailing edge ofa binary function.

[0016] The apparatus of the present invention can further comprise meansfor moving the sample from a heating position of the heating means to adetection position of the detecting means, and control means for turningoff the heating means heating the sample before detection by thedetecting means. When these means are provided, infrared light emittedfrom the invisible symbol can be detected in a process of cooling thesample.

[0017] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0018] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate presently preferredembodiments of the invention, and together with the general descriptiongiven above and the detailed description of the preferred embodimentsgiven below, serve to explain the principles of the invention.

[0019]FIG. 1 is a block diagram showing an invisible symbol readingapparatus of Example 1;

[0020]FIG. 2 is a graph showing the intensity of infrared light emittedfrom an invisible symbol measured in Example 1;

[0021]FIG. 3 is a graph for explaining a method of determining peakvalues from differential data of measured data in Example 1;

[0022]FIG. 4 is a graph showing differential data and a binary functionin Example 1;

[0023]FIG. 5 is a graph showing a binary function and data obtained byoptimizing the widths of bars and spaces;

[0024]FIG. 6 is a block diagram showing an invisible symbol readingapparatus of Example 2;

[0025]FIG. 7A is a perspective view of a stage and a target used in theinvisible symbol reading apparatus of Example 2, FIG. 7B is aperspective view of the target, and FIG. 7C is a sectional view of thestage and the target;

[0026]FIG. 8A is a plan view showing an invisible symbol readingapparatus of Example 5 and FIG. 8B is a front view of the apparatus;

[0027]FIG. 9 is a view showing the construction of an invisible symbolreading apparatus of Example 6;

[0028]FIG. 10 is a perspective view showing the positional relationshipbetween a tubular halogen lamp with reflector and a stage top plate inthe invisible symbol reading apparatus;

[0029]FIG. 11 is a sectional view of the tubular halogen lamp withreflector of the invisible symbol reading apparatus of Example 6;

[0030]FIG. 12 is a graph showing infrared light intensity emitted from ameasured barcode in Example 6;

[0031]FIG. 13 is a view showing the operation of a thermal head in aninvisible symbol reading apparatus of Example 7;

[0032]FIG. 14 is a plan view showing a barcode information portion andits peripheral portion;

[0033]FIG. 15 is a block diagram showing an invisible symbol readingapparatus of Example 10;

[0034]FIGS. 16A and 16B are graphs showing infrared emission signalsobtained by different spatial resolutions;

[0035]FIG. 17 is a perspective view showing an arrangement of adetecting optical system, heating means, and conveyor means of aninvisible symbol reading apparatus of Example 12; and

[0036]FIG. 18 is a view showing another arrangement of the detectingoperation system, heating means, and conveyor means of the invisiblesymbol reading apparatus of Example 12.

DETAILED DESCRIPTION OF THE INVENTION

[0037] The present invention will be described in more detail below.

[0038] In the present invention, symbols mean linear (one-dimensional)barcodes and two-dimensional symbols. Reading of a barcode as arepresentative symbol will be principally described below.

[0039] The principle of the present invention is to use infrared lightemitted from an invisible barcode as reading light. Every molecule hasabsorption in the infrared region as interaction with its molecularvibration. Therefore, every molecule absorbs infrared light having awavelength intrinsic to its molecular structure and state and emitsinfrared light. That is, the wavelength of infrared light as a result ofinteraction changes in accordance with the molecular structure or stateof a molecule. Accordingly, the influence of an interfering substancecan be suppressed by using a material not used in a sample (an articleto which a barcode is attached) as the material of the barcode.Additionally, a material containing neither metals nor halogens andfound to be safe both when in use and wasted can be chosen as thematerial of a barcode. When a barcode is to be read with infrared light,the energy per photon is smaller than that of ultraviolet light orvisible light, so the operation is readily influenced by ambient heat.By contrast, the present invention can effectively remove ambientthermal noise.

[0040] That is, an invisible symbol reading method of the presentinvention comprises the steps of heating an invisible symbol formed on asample and containing a material which emits infrared light when heated,detecting infrared light emitted from the invisible symbol, calculatinga differential coefficient of a detection signal corresponding to aposition on the sample, determining, on the basis of upper and lowerthreshold values set for the differential coefficient, a maximum valueof the differential coefficient in a region exceeding the upperthreshold value and a minimum value of the differential. coefficient ina region smaller than the lower threshold value, and binarizing thedetection signal by using the maximum or minimum value as a leading ortrailing edge of a binary function.

[0041] In the present invention, an invisible symbol formed on a sampleand containing a material which emits infrared light when heated isheated, and infrared light emitted from the invisible symbol as a resultof heating is detected.

[0042] If the heating means exists near the sample while a signal isdetected, heat from this heating means is detected as a noise, and thisdecreases the sensitivity. Also, when a signal is detected while thesample is heated, heat absorbed as infrared light by the barcodediffuses to the underlying substrate. This decreases the signal contrastbetween the barcode and the substrate. In the present invention,therefore, it is also possible to heat a sample on which an invisiblebarcode is formed and detect infrared light emitted from the barcode inthe course of cooling the sample.

[0043] More specifically, heating means is installed apart fromdetecting means, and a sample is moved from the heating position of theheating means to the detection position of the detecting means.Alternatively, the heating means heating a sample is turned off beforedetection by the detecting means. When a method like this is used todetect infrared light emitted from a barcode during the course of samplecooling, thermal noise from the heating means can be reduced, and anydecrease of contrast between the barcode and the substrate can beprevented.

[0044] Next, the detection signal obtained by the detecting means isbinarized. This detection signal of a barcode is a curve correspondingto bars (black portions in common barcodes) and spaces. If there is noinfluence of thermal noise, it is possible to set an appropriatethreshold value, regard portions larger and smaller than this thresholdvalue to be bars and spaces, respectively, and determine pointsintersecting the threshold value as the start and end positions of eachbar. However, even when thermal noise from the heating means is reducedas described above, ambient thermal noise is superposed on the detectionsignal of infrared light, so the whole detection signal oftenfluctuates. Hence, this simple method cannot binarize the signal.

[0045] In the present invention, the detection signal is differentiated,and maximum and minimum values of the differential coefficient are usedto determine the start and end positions of a bar. More specifically,two threshold values, i.e., upper and lower threshold values are setwith a predetermined width between them with respect to the differentialcoefficient of the detection signal. A maximum value of the differentialcoefficient in a region exceeding the upper threshold value and aminimum value of the differential coefficient in a region smaller thanthe lower threshold value are determined. The detection signal isbinarized by using these peak values as the leading or trailing edge ofa binary function. Note that the detection signal is desirably smoothedto reduce the influence of thermal noise before being differentiated. Itis further desirable to smooth the slope in calculating the differentialcoefficient.

[0046] In actually used barcodes, the widths of bars (or spaces) areintegral multiples of a basic width. However, the binary functionobtained by the above method often shifts from an integral multiple ofthe basic width owing to the influence of noise or smoothing. Therefore,it is desirable to perform data processing of correcting the obtainedbinary function into an integral multiple of the basic width. When thedata thus obtained is decoded in the same manner as for a commonbarcode, information recorded in the invisible barcode can be read.

[0047] The reading accuracy can be increased by using various rulesapplied to barcodes. For example, assuming a bar with the basic width is“1” and a space with the basic width is “0”, a JAN code of eightcharacters is represented in accordance with the following rule. Thatis, a JAN code of. eight characters is composed of left guard bars“101”, four data characters on the left side, center bars “01010”, threedata characters on the right side, one modular check character, andright guard bars “101”. One character (number) is expressed by forming amodule seven times as large as the basic width by using two bars and twospaces. The boundary between characters coincides with a change from abar to a space. The modular check character is used to check for a readerror and takes a value calculated on the basis of the data characters.

[0048] An apparatus of the present invention can increase the readingaccuracy by checking whether the result of binarization meets theaforementioned rules. The apparatus can also correct the result ofbinarization to eliminate contradiction. Details of the data processingmethod will be described later in examples of the present invention.

[0049] U.S. Pat. No. 5,294,198 discloses a semiconductor deviceevaluation apparatus for obtaining surface information of a sample bydetecting infrared light emitted by heating. This apparatus observesinfrared light emitted from different portions on the surface of adevice and evaluates whether the device is normal or abnormal on thebasis of the infrared emission (temperature). In this evaluation, theapparatus checks whether the deviation of each measured value from areference value exceeds a predetermined value. However, an invisiblesymbol such as an invisible barcode as an object of the presentinvention cannot be read by simply comparing the measured value with thereference value as in this apparatus.

[0050] Jpn. Pat. Appln. KOKAI Publication No. 7-282184 describes anapparatus for sensing infrared light emitted by heating to discriminatean invisible symbol for examining the genuineness of an article. In thisapparatus, a heat roller heats an invisible symbol formed on the surfaceof a card by using a heat-absorbing substance, and an array sensorsenses the thermal emission of light. The apparatus examines thegenuineness of the card on the basis of matching between the magneticinformation and the light-emitting invisible symbol. Unfortunately, anobject of detection by this prior art is a genuineness examination marklarger than general barcodes. To read symbols such as barcodes as in thepresent invention, an optical system for detecting emission of infraredlight with spatial resolution meeting the minimum width of a barcode isimportant. However, Jpn. Pat. Appln. KOKAI Publication No. 7-282184 doesnot describe any optical system for guiding light emission to aninfrared sensor such as an array sensor. Hence, the apparatus cannot beapplied to reading of barcodes.

[0051] Materials used in the present invention and components of areading apparatus will be described in more detail below.

[0052] In the present invention, a compound having an infraredabsorption wavelength far apart from that of a sample (substrate) isused as the material of an invisible barcode. Since infrared light isabsorbed by atmospheric moisture or carbon dioxide depending on thewavelength, the intensity of a detection signal can largely vary in somecases. The detection signal is also readily influenced by water orcontamination on the surface of an invisible barcode. Accordingly, it isimportant to eliminate these influences. It is, therefore, desirable touse a compound having a cyano (CN) group as the material of an invisiblebarcode, and the use of a polymer containing a cyano group is moredesirable. This material is used in the form of ink-jet printer ink,thermal transfer ink ribbon, electrophotographic toner, or fiber to forman invisible barcode on a sample made of paper, polymer, cloth, or thelike. For example, when an invisible barcode made from a polymercontaining a cyano group is heated, the barcode emits infrared lightwith a wavelength of about 4.5 μm.

[0053] To obtain emission of infrared light from an invisible barcode byexciting molecular vibration of the barcode, a barcode containing amaterial which absorbs infrared light is heated. The heating means isdesirably a contact type heater such as a thermal head, thermal bar, orhot stamp; a warm air heater; or a halogen lamp which emits infraredlight in a broad wavelength range. To excite the molecular state of abarcode, it is also possible to irradiate light having a specificwavelength. However, this method is undesirable in terms of efficiencybecause light except for the specific wavelength is cut.

[0054] In the present invention, the heating means is preferably capableof heating the entire area of a barcode information portion at once.When a halogen lamp is used as this heating means, for example, atubular halogen lamp is selected, and a bifocal reflector having anelliptic section is installed around the lamp. The lamp is positioned atone focal point of the reflector, and a sample is positioned at theother focal point. The reflector linearly condenses infrared light fromthe halogen lamp onto the sample and heats the sample with this lightwithout contacting the sample. When the output of the halogen lamp isabout 1 kW, however, a sample may be overheated to 100° C. or more in afew seconds if the sample is positioned at the focal point of thereflector. To prevent this, it is preferable to install means capable ofadjusting the vertical position of the halogen lamp and place a samplein a position shifted a few mm from the focal point of the reflector. Itis also preferable to heat a sample to a fixed temperature bydetermining the heating time and current value by measuring thetemperature of the sample by a radiation thermometer and install asafety device for protecting the sample from overheating.

[0055] For a sample unsuited to being heated to a high temperature,e.g., a card in which magnetic information is written, it is necessaryto heat only the barcode portion and hold the card main body at a lowtemperature. To this end, a heater is desirably brought into contactwith the barcode printed surface to heat it.

[0056] When any of these heating means is used to heat a barcode,excessive heating of the surroundings causes thermal noise. Therefore,it is preferable to heat a barcode by adjusting not only the ultimatetemperature but also the way the temperature is changed in accordancewith each sample.

[0057] From the viewpoint of the read accuracy, the appropriate heatingtemperature of a sample has a certain relation to the sensitivity of adetector. That is, when the sensitivity of a detector is high, it isdesirable to minimize the heating temperature of a sample to reduceunnecessary thermal noise. For example, when an MCT detector is used aswill be described later, the heating temperature of a sample ispreferably 50 to 100° C., and more preferably 70 to 80° C.

[0058] When a heated sample is moved from the heating position to thedetection position so as to be put in the cooling process as describedearlier, the moving direction can be either perpendicular or parallel tothe scan direction of a barcode. Examples of the moving means are astepping motor and conveyor rollers. It is also possible to use a hotstage having a built-in heating means such as a bar heater and turn offthe heating means before a heated sample is subjected to detection bydetecting means. This hot stage preferably has a mechanism capable ofradiating heat and cooling. In order to make a sample brought intocontact with the hot stage uniformly in plane, it is desirable to use atarget described below.

[0059] Infrared light emitted from a barcode is invisible, so it isdifficult to align the optical axis of a detecting optical system with abarcode information portion on a sample. To allow easy alignment,therefore, it is also possible to use a target for alignment and align abarcode region with this target. For example, a target obtained byforming a cross-shaped mark matching the optical axis of the opticalsystem on a transparent film is used. Alternatively, a target havingmarks in three to four portions of a frame is used to align theintersection of extension lines of the marks with the optical axis ofthe optical system. Visible light can also be irradiated as guide lightto attain alignment with the optical axis of the optical system. As alight source of this visible light, a low-output diode laser with awavelength of about 650 nm can be used.

[0060] To detect infrared light emitted from a sample having a barcodeon it in accordance with the position on the sample, the optical axis ofthe optical system and the sample are moved relative to each other. Twomethods are possible for this purpose: one is a method of scanning theoptical axis on the sample by rotating an optical element, and the otheris a method of conveying the sample by a conveyor mechanism. The anglethe optical axis of the optical system makes with a bar of a barcodesometimes changes in accordance with the position, and this may changethe apparent bar width to be read. To prevent this, it is desirable toeliminate the dependence of the bar width on the position by positioninga sample at the focal point on the optical axis of the optical systemand conveying the sample while a fixed angle is held between the opticalaxis and the sample. It is particularly desirable that the optical axisof the optical system and the sample surface be perpendicular to eachother. Note that if the apparent bar width changes in accordance withthe position, data correction is performed. To convey a sample by theconveyor mechanism, the way the sample is moved is adjusted inaccordance with a barcode. Since the minimum bar width (basic width) ofa common barcode is about 250 μm, the conveyance step is preferably 100μm or less, and more preferably about 10 μm. When a detector composed ofa satisfactorily large number of elements, e.g., an EPA (Focal PlaneArray) is used, pieces of information concerning different portions of asample can be obtained at once. This eliminates the need to convey thesample to read signals.

[0061] A mirror or a lens is used as an optical element for focusing andguiding infrared emission from a sample to the detector. When the objectto be detected is a common barcode whose minimum width is about 250 μm,a proper optical element is chosen in accordance with the size of anelement of the detector. When the size of each element constructing thedetector is 100 μm or less, a barcode can be detected withsatisfactorily high spatial resolution, so the optical element can beeither a mirror or a lens. If the size of each element constructing thedetector is larger than 100 μm, it is necessary to enlarge an imagebefore image formation. Hence, the use of a lens or a combined mirror isdesirable. If this is the case, a Cassegrain lens used in a microscopicoptical system is desirable, and a lens with a large work length is moredesirable. The material of the lens is so selected as to meet thewavelength of infrared light to be detected. If visible light is used asguide light, a material which transmits both visible light and infraredlight is chosen as the lens material. An optical stop is desirablyinserted on the optical axis of the optical system to improve thequality of light reaching the detector. If light beams having adifferent wave length from each other are incident to a refractiveoptical element, they pass different optical paths due to differencebetween focal lengths. Therefore, the use of the refractive opticalelement and the optical stop is advantageous because infrared light witha specific wavelength can be detected.

[0062] A method of selecting a wavelength can be used to reduce ambientthermal noise and detect infrared emission from a sample. To select awavelength, a grating or a filter can be used. A grating can extract awavelength in a narrow range, but the utilization (throughput) of lightis low, and the apparatus is enlarged. A filter is obtained byperforming appropriate optical processing for a substrate suited to awavelength to be used and hence can be used easily. High-pass, low-pass,and bandpass filters can be selectively used. Infrared emission from abarcode shows peaks centering around a wavelength due to molecularvibration, whereas ambient thermal noise is independent of wavelength.By using a bandpass filter having a central wavelength corresponding tothe wavelength of infrared emission from a barcode, it is possible toeffectively remove thermal noise and selectively guide the infraredemission from the barcode to the detector. For example, when a barcodeis formed by using polyacrylonitrile containing a cyano group, thewavelength of infrared emission is around 4.5 μm, so a bandpass filterwhich transmits this wavelength is used. Bandpass filters are classifiedinto a wide-bandpass filter (10% or more of the central wavelength) anda narrow-bandpass filter (2% to 10% of the central wavelength) inaccordance with the band width. The narrower the band width, the higherthe efficiency of thermal noise removal, but the smaller the transmittedlight intensity. Hence, it is desirable to use a bandpass filter havingan appropriate band width in accordance with the sensitivity and S/Nratio of the detector.

[0063] As another method of reducing ambient thermal noise and detectinginfrared emission from a sample, a method of optically modulatinginfrared emission and detecting the phase by using a lock-in amplifieris also effective. To optically modulate infrared emission, it ispossible to use a method using an optical chopper, a tuning-fork choppera polygon mirror or a galvano-mirror or a method which performspolarization modulation by additionally using a polarizing element. Toavoid distortion of signals and reduction of the light intensity, theuse of an optical chopper or a tuning-fork chopper is desirable. Themodulation frequency is desirably 1 Hz to 100 kHz, and more desirably 10Hz to 10 kHz.

[0064] To read a barcode, infrared emission can also be subjected to ACcoupling amplification. When a sample having a barcode is conveyed at arate at which the basic width of bars can be scanned in a time notexceeding the reciprocal of the cutoff frequency of an AC couplingamplifier (e.g., a time of 200 ms or less if the cutoff frequency is 5Hz), infrared emission can be amplified without being influenced byambient thermal noise. If the intensity of a signal whose phase is to bedetected by a lock-in amplifier is low, a preamplifier is preferablyinstalled before the lock-in amplifier to amplify the signal intensityby 10 to 100 times.

[0065] When a barcode is read by spatial resolution equivalent to thebasic width of the barcode, the read time can be reduced by increasingthe conveyance rate of a sample, but the apparatus function issuperposed (convoluted) on the amplitude of a signal. If this is thecase, it is desirable to use a filter as a function of the signalfrequency on the signal to remove (deconvolute) the apparatus functionand correct the amplitude and then binarize the signal.

[0066] As an infrared detector, a high-sensitivity detector having asensitivity region meeting infrared emission from a barcode is used.When a barcode is formed by a polymer containing a cyano group, it isdesirable to use a detector having an element made from MCT (MercuryCadmium Tellurium), InSb (indium antimony), or PtSi (platinum silicide),each of which has high sensitivity near 4.5 μm. Any of these detectorsis a quantum detector which detects infrared emission as light, so thedetector is cooled to a low temperature to reduce thermal noise from thedetecting element itself. The cooling means can be any of cooling usingliquid nitrogen, electronic cooling using a Peltier element, Stirlingcooling using a compressor, a pulse-tube cooling, and J-T(Joule-Thomson) cooling using adiabatic expansion. To perform cryogeniccooling, the use of liquid nitrogen, Stirling cooling or a pulse-tubecooling is desirable. Note that a detector such as a bolometer whichdetects infrared emission as heat requires no cooling and hence can besuitably used provided that the system generates intense signals or thedetector has high sensitivity.

[0067] To distinguish between a signal from a barcode and thermal noise,it is preferable to regard the signal level of the underlying substrateas the signal level of background and correct a measured detectionsignal on the basis of this signal level. It is also possible to correcta measured detection signal by regarding the average signal level in abroad range including both an information portion and a peripheralportion (substrate) as the signal level of background. In this method,however, the background signal level is estimated to be higher than theactual level, so the contrast between the information portion and theperipheral portion lowers when correction is performed. Therefore, it isdesirable to obtain the signal level of only the peripheral portion. Tothis end, a signal from the peripheral portion can be detected in adifferent step from the step of detecting a signal from the barcodeinformation portion.

[0068] Also, the signal level of the peripheral portion is preferablyelectrically corrected by AC coupling amplification as follows. That is,the detection position is so moved as to alternately reciprocate over aninformation portion and a peripheral portion of a barcode across theedge of the barcode in a time not exceeding the reciprocal of the cutofffrequency of an AC coupling amplifier. The moving range is about 1 toten-odd times the spot diameter from the edge of the barcode informationportion. A signal change caused by this reciprocal motion can beseparated, in accordance with the frequency, from a signal changeresulting from conveyance of the barcode in the scan direction.Consequently, the detection position is preferably reciprocated at arate 10 times the conveyance rate or more. This method can remove evenslight thermal noise by correction using the signal level of theperipheral portion.

EXAMPLES

[0069] Examples of the present invention will be described below.

Example 1

[0070] An acrylonitrile (25%)-styrene (75%) copolymer (AS resin) wasused as the material of an invisible barcode. This resin was pulverizedto have an average size of 11 μm to prepare toner not containingpigments. This toner was used as toner of a laser beam printer to forman invisible linear barcode on plain paper. The formed barcodecorresponds to an enlarged size with a basic width of 3 mm obtained byenlarging a JAN code with a basic width of 300 μm printed on an existingarticle selected at random.

[0071]FIG. 1 is a block diagram showing an invisible symbol readingapparatus used in this example. A sample 1 on which the invisiblebarcode is printed is held on a pulse stage 11. This pulse stage 11moves in accordance with a signal from a stage controller 12. The sample1 is heated by warm air blown from a warm air heater 13. This heatingexcites molecular vibration of a cyano group in the invisible barcode,and infrared emission occurs near 4.5 μm accordingly.

[0072] This infrared emission is reflected by an elliptic mirror 15through an optical chopper 14 and detected by an MCT detector 17 througha bandpass infrared filter 16. The elliptic mirror 15 has a focal lengthof 100 mm and forms an image without changing the magnification. The MCTdetector 17 has a highest-sensitivity wavelength of 4.5 μm, and itslight-receiving surface is composed of square elements of 1 mm side.This MCT detector 17 is electronically cooled by a Peltier element andused at −60° C. A bias power supply 18 supplies power to the MCTdetector 17. The MCT detector 17 converts a change in its electricalresistance caused by infrared light into a voltage and thereby generatesa detection signal.

[0073] A preamplifier 19 amplifies the output from the MCT detector 17by 100 times, and a lock-in amplifier 20 detects and amplifies thein-phase signal. A digital sampling oscilloscope (not shown) triggeredby an output from the optical chopper displays the waveform of thedetection signal. The detection signal is input to an A/D conversionboard of a personal computer 21 and subjected to data processing. (to bedescribed later). A decoder 23 decodes the processed signal.

[0074] The operation was actually performed as follows. The sample 1 washeld on the pulse stage 11 by a plate-like magnet and so adjusted thatthe invisible symbol region on the sample was positioned at the focalpoint of the optical system. The warm air heater 13 was so installed asto blow warm air against the sample 1. The position of blow of warm airwas set upstream of the focal point (the position moved closer to thefocal point when the stage moved), and a signal was detected. As aconsequence, the signal intensity increased when the signal was measuredby heating a position about 3 mm from the detection position by blowingwarm air. Infrared emission at the focal point was detected while thepulse stage 11 was moved at a fixed rate by the signal from the stagecontroller 12. The movement of the pulse stage 11 was monitored by anoptical sensor and measured by taking margins before and after theinvisible barcode. Data of the detection signal was input as a file tothe personal computer 21.

[0075]FIG. 2 shows a detection signal when the sample was heated to 70°C. In this detection signal, peaks and valleys are formed in accordancewith bars and spaces, and the widths of the peaks (valleys) change inaccordance with the widths of the bars (spaces). However, ambientthermal noise is superposed on the detection signal, so the signal wavesas a whole. This makes it impossible to apply the method which sets anappropriate threshold value, regards portions larger and smaller thanthis threshold value as bars and spaces, respectively, and determineintersections to the threshold value as the start and end positions ofeach bar.

[0076] Hence, the measured data was processed as follows. First, thedata was smoothed by using the moving average method to removehigh-frequency noise. This high-frequency noise was removed when therating of smoothing was 41 or more, for example, as a basic widthcorresponds with about 100. Next, the detection signal wasdifferentiated for binarization. To remove the influence of thermalnoise, the slopes at surrounding points were smoothed to calculate adifferential coefficient y′.

[0077]FIG. 3 is an enlarged view showing a partial change in thedifferential coefficient y′ (ordinate) corresponding to the position(distance on the abscissa) on the sample. As shown in FIG. 3, twoadequate threshold values were set for y′. An extreme value (maximumvalue) of y′ in a region exceeding the upper threshold value and anextreme value (minimum-value) of y′ in a region smaller than the lowerthreshold value are candidates of the start and end positions of a bar(space). Peaks between the two threshold values were removed byregarding them as noise. Note that changing the threshold values by amagnitude of 5% or less of the amplitude (difference between the maximumand minimum values) had no influence on the results.

[0078] Bars and spaces alternately appear in an actual barcode, so thepeaks of y′ are also supposed to alternately appear above and below thethreshold values. However, two peaks (peaks in ranges B and C in FIG. 3)can appear in a region smaller than the lower threshold value owing tothe influence of thermal noise. If this is the case, real peaks aredetermined following a procedure below. First, a minimum value of y′appearing for the first time in a region smaller than the lowerthreshold value is regarded as a provisional minimum peak. As in a rangeA, if y′ does not decrease after the provisional minimum peak and amaximum peak appears in a region exceeding the upper threshold value,the provisional minimum peak is considered to be a real minimum peak. Onthe other hand, as in the ranges B and C, if y′ again decreases afterthe provisional minimum peak and before exceeding the upper thresholdvalue and a minimum peak appears in the region smaller than the lowerthreshold value, these two minimum values in the ranges B and C arecompared, and a smaller one is considered to be a real minimum peak. Areal maximum peak is determined following the same procedure. Addressesx(i) of the leading and trailing edges of a binary functioncorresponding to a peak value y′(i) thus determined are the start andend positions of an actually measured bar. FIG. 4 collectively showsdata of the differential coefficient y′ and a binary function whichrises and falls in the start and end positions, respectively, of a bar.

[0079] The width of a bar or a space is supposed to be obtained when thedifference between two continuous addresses x(i+1) and x(i) iscalculated. However, in this calculation the width of a bar or a spacetended to be smaller than an integral multiple of the basic width underthe influence of smoothing. In contrast, the total width of an adjacentbar and space was an integral multiple of the sum of the basic widths ofthe two. A detailed calculation procedure is as follows.

[0080] The difference between start addresses (or end addresses) x(i+2)and x(i) of adjacent bars is calculated as a width X(i). This X(i)corresponds to the total width of an adjacent bar and space. A pluralityof X(i)'s are sorted and arranged in ascending order. A half value ofthe average of two smallest X's is calculated as an initial value of abasic width W. A value 2.5 times this W is used as a threshold value,and X's assumed to have a width W₂ which is twice the basic width areselected from X's equal to or smaller than the threshold value. Theaverage value of these X's is calculated as new W. This new W and theinitial value are compared, and the process is repeated until the twovalues are equal. W finally obtained by this operation is regarded as asecond initial value of W. A value 3.5 times this second W is used as athreshold value, and X's assumed to have a width W₃ which is three timesthe basic width are selected from X's equal to or smaller than thethreshold value. The average value of these X's is calculated as new W.This new W and the second initial value are compared, and the process isrepeated until the two values are equal. W finally obtained by thisoperation is regarded as a third initial-value of W. In the same manneras above, a threshold value is calculated by using W finally obtained inthe immediately preceding calculation as a new initial value. Similarcalculations are repeatedly performed for W₄ and W₅ to obtain W as afifth initial value. The value of each X(i) is divided by the fifthinitial value W and rounded to obtain an integer. W is again calculatedon the basis of X's from X assumed to have the width W₂ to X assumed tohave the width W₅. This new W and the fifth initial value are compared,and the process is repeated until the two values are equal. In thismanner a final basic width W is obtained.

[0081] The positions of bars and spaces are determined in units of theobtained basic width W. To align the start position of the barcode whilecorrecting any offset caused by noise, let the entire offset be d andthe corrected value of W be ω. While ω is changed in units of 0.001Wfrom 0.99W to 1.01W and d is changed in units of 0.01ω from −0.5ω to0.5ω for certain ω, summation Σδ of differences δ between a (bar startor end address) and an (integral multiple of ω) is calculated as per${\sum\delta} = {\sum\limits_{i = 0}^{n}{{{x(i)} - {{{INT}\left( {\frac{{x(i)} - d}{\omega} + 0.5} \right)} \times \omega} + d}}}$${y(i)} = {{INT}\left( {\frac{{x(i)} - d}{\omega} + 0.5} \right)}$

[0082] Σδ for different combinations of ω and d are compared, and acombination of ω and d by which Σδ is a minimum is determined.

[0083] The bar start or end address x(i) is corrected by the offset dand represented by an integral multiple y(i) of ω (an integer isobtained by rounding). A bar or space width Y(i) is calculated from thedifference between the integral addresses y(i). FIG. 5 shows dataobtained by optimizing the final bar or space width thus obtained and abinary function. This optimized data was decoded by the decoder 23.Consequently, the decoded data matched the result of decoding ofenlarged data of the original data printed on the existing article.

Example 2

[0084] Toner made from the same AS resin as used in Example 1 was usedas toner of a laser beam printer to form an invisible barcode on plainpaper. The formed barcode corresponds to a standard-size barcode with abasic width of 300 μm printed on an existing article selected at random.

[0085]FIG. 6 is a block diagram showing an invisible symbol readingapparatus used in this example. A holder 32 having a built-in bar heater31 holds a sample 1 on which the invisible barcode is printed on a pulsestage 11. A signal from a built-in thermocouple (not shown) of theholder 32 is input to a temperature controller 33 to adjust the currentto be supplied to the bar heater 31, thereby heating the sample 1 to apredetermined temperature. A target 34 is used to align the optical axisof an optical system and a barcode formation region of the sample 1. Asshown in FIG. 7A or 7B, this target 34 is a frame-like member, and threeor four marks 34 a are formed on it. As shown in FIGS. 7A to 7C, theintersection of extension lines of the marks 34 a is aligned with theoptical axis of the optical system. The target 34 functions as a keepplate to hold down the sample 1 on the holder 32.

[0086] An MCT detector 17 detects infrared emission from the invisiblebarcode through an optical chopper 14, a Cassegrain lens 35, and abandpass infrared filter 16. An optical stop (not shown) is installed onthe optical axis. The Cassegrain lens 35 has a focal length of 13 mm andforms an enlarged image of ×15. The MCT detector 17 has ahighest-sensitivity wavelength of 4.5 μm, and its light-receivingsurface is composed of square elements of 1 mm side. This MCT detector17 is electronically cooled by liquid nitrogen and used at −200° C. Abias power supply 18 supplies power to the MCT detector 17.

[0087] As in Example 1, a preamplifier 19 amplifies the output from theMCT detector 17 by 100 times, and a lock-in amplifier 20 detects andamplifies the phase of the signal. A digital sampling oscilloscope (notshown) triggered by an output from the optical chopper displays thewaveform of the detection signal. The detection signal is input to anA/D conversion board 22 of a personal computer 21 and subjected to dataprocessing (to be described later). A decoder 23 decodes the processedsignal.

[0088] The operation was actually performed as follows. The sample 1 washeld on the pulse stage 11 by a plate-like magnet and so adjusted thatthe invisible barcode region on the sample was positioned at the focalpoint of the optical system. The sample 1 was heated to 70° C. under thecontrol of the temperature controller 33. Infrared emission at the focalpoint was detected while the pulse stage 11 was moved at a fixed rate bythe signal from the stage controller 12. The start and end positions ofthe barcode were measured by taking margins for these positions by usinglimit switch signals from the pulse stage 11. Data of the detectionsignal was input as a file to the personal computer 21.

[0089] Following the same procedures as in Example 1, the measured datawas smoothed and binarized by differentiation, and the bar and spacewidths were optimized. This optimized data was decoded by the decoder23. Consequently, the decoded data matched the result of decoding of theoriginal data printed on the existing article.

Example 3

[0090] A polyacrylonitrile powder was dispersed in a 5 wt % aqueouspolyvinyl alcohol solution at a ratio of 2 wt % with respect topolyvinyl alcohol. The resultant dispersion was used as ink of an inkjet printer to form an invisible barcode on plain paper. The formedbarcode corresponds to the same standard-size barcode with a basic widthof 300 μm as in Example 2.

[0091] A reading apparatus shown in FIG. 6 was used to heat a sample 1to 70° C. while a pulse stage 11 was moved at a fixed rate. Infraredemission at a focal point was detected and input to a personal computer21. The start and end positions of the barcode were measured by takingmargins for these positions by using limit switch signals from the pulsestage 11. Following the same procedures as in Example 1, the measureddata was smoothed and binarized by differentiation, and the bar andspace widths were optimized. This optimized data was decoded by adecoder 23, the decoded data matched the result of decoding of theoriginal data.

Example 4

[0092] An acrylonitrile-styrene copolymer (AS resin) was dissolved intoluene, and the solution was mixed with wax. A substrate was coatedwith the resultant material and dried to form a heat-sensitive inkribbon. This heat-sensitive ink ribbon was used to form an invisiblebarcode on plain paper. The formed barcode corresponds to the samestandard-size barcode with a basic width of 300 μm as in Example 2.

[0093] A reading apparatus shown in FIG. 6 was used to heat a sample 1to 70° C. while a pulse stage 11 was moved at a fixed rate. Infraredemission at a focal point was detected and input to a personal computer21. The start and end positions of the barcode were measured by takingmargins for these positions by using limit switch signals from the pulsestage 11. Following the same procedures as in Example 1, the measureddata was smoothed and binarized by differentiation, and the bar andspace widths were optimized. This optimized data was decoded by adecoder 23, and the decoded data matched the result of decoding of theoriginal data.

Example 5

[0094]FIGS. 8A and 8B show details of the main parts of the readingapparatus according to the present invention. FIG. 8A is a plan view,and FIG. 8B is a front view. A lens barrel 101, an MCT detector 102, anda liquid nitrogen tank 102A for cooling vertically extend above thesample surface. A reflecting objective lens 103 and a bandpass infraredfilter 104 are arranged on the optical axis of an optical system. Animage of infrared light from the sample is formed on the light receivingsurface of the MCT detector 102. The focal point of the optical systemis adjusted by a focal point adjusting screw 106. The MCT detector 102inputs a detection signal to a preamplifier 107.

Example 6

[0095] An acrylonitrile (25%)-styrene (75%) copolymer (AS resin) wasused as the material of an invisible barcode. This resin was pulverizedto have an average size of 11 μm to prepare toner not containingpigments. This toner was used as toner of a laser beam printer to forman invisible barcode on label paper. The formed barcode corresponds to abarcode with a basic width of 300 μm printed on an existing articleselected at random. The label paper on which the invisible barcode wasformed was pasted on a plastic card to form a sample (A).

[0096]FIG. 9 shows the construction of a barcode reading apparatus usedin this example. FIG. 10 is a perspective view showing the positionalrelationship between a stage for holding a sample and a tubular halogenlamp with reflector in this barcode reading apparatus. FIG. 11 is asectional view of the tubular halogen lamp with reflector. The barcodereading apparatus of this example will be described in more detail belowwith reference to FIGS. 9 to 11.

[0097] Rollers 201 are placed below a frame 200, and a pulse stage 202and a stage top plate 203 are mounted on these rollers 201. A sample 1on which the invisible barcode label paper is pasted is placed on thestage top plate 203 and aligned with reference to a target 204. Thesecomponents are moved between a detection position (indicated by thesolid lines) and a heating position (indicated by the alternate long andshort dashed lines) along a direction L shown in FIG. 10 by an air valve(not shown). The pulse stage 202 moves in a direction S (barcode scandirection) shown in FIG. 10 under the control of a stage controller (notshown). A micrometer 205 adjusts the height of the stage top plate 203.

[0098] A tubular halogen lamp 206 with a reflector 207 is so installedas to be positioned above the sample when the sample moves to theheating position. As shown in FIG. 11, the section of the reflector 207forms a part of an ellipse. The halogen lamp 206 is arranged at thefirst focal point of the reflector 207 and installed in a lamp house.Light from the halogen lamp 206 is linearly condensed to have a width Dat a second focal point F2. The lamp house includes a mechanism (notshown) for adjusting its vertical position. To protect the sample fromoverheating, this mechanism adjusts its vertical position so that theupper surface of the sample deviates 2 to 6 mm from the focal point ofthe reflector 207. A radiation thermometer (not shown) detects thetemperature on the upper surface on the sample. If the temperatureexceeds a set value, a safety device sends an alarm signal to the powersupply of the halogen lamp 206 to cut off the switch.

[0099] An optical chopper unit is attached to the frame 200. In thisunit, a motor 208 is mounted facing down, and an optical chopper 209 isheld by this motor 208 so as to be rotatable above the sample in thedetection position. An optical sensor 210 measures the rotating speed ofthe optical chopper 209. A frame 211 for safety is formed around theoptical chopper 209. A window is formed in this frame 211 to allowinfrared emission from the sample to reach a detecting optical system.The frame 200 also holds a lens barrel 101 of the detecting opticalsystem including a calcium fluoride lens 113, a bandpass infrared filter104, and an MCT detector 102 above the optical chopper 209. The calciumfluoride lens 113 has a diameter of 25 mm and a focal length of 50 mm.The focal point of the optical system is adjusted by a focal pointadjusting screw 106. Although not shown, an optical stop is arranged onthe optical axis of the detecting optical system.

[0100] A barcode is read by using the above reading apparatus asfollows. First, the pulse stage 202 is set in the detection position,and a sample is placed on the stage top plate 203. The optical axis ofthe detecting optical system and a barcode information portion of thesample 1 are aligned with reference to the target 204. That is, the endof a left margin of the barcode is positioned on the optical axis of thedetecting optical system. This position is an initial position of thesample 1. Next, the air valve is activated to move the sample 1 togetherwith the stage top plate 203 to the heating position. The halogen lamp206 heats the upper surface of the sample 1 to 75° C. in the initialposition. This heating excites molecular vibration of a cyano groupcontained in the invisible barcode material on the sample 1, andinfrared emission near 4.5 μm occurs accordingly. After heating, thestage top plate 203 is immediately returned to the initial position, andthe infrared emission is detected in a cooling process as follows.

[0101] A stage controller (not shown) supplies a signal to scan thepulse stage 202 at a fixed rate of 20 mm/sec in the direction S. Theinfrared emission from the sample 1 is condensed by the calcium fluoridelens 113 through the optical chopper 209 and detected by the MCTdetector 102 through the bandpass infrared filter 104. The calciumfluoride lens 113 has a diameter of 25 mm and a focal length of 50 mm.The distance between the lens and the detector is 2.5 times(magnification is ×2.5) the distance between the lens and the sample.The MCT detector 102 has a highest-sensitivity wavelength of 4.5 μm, andits light receiving surface is composed of square elements of 1 mm side.When in use, this MCT detector 102 is cooled to −200° C. by liquidnitrogen 102A. A bias power supply (not shown) supplies power to the MCTdetector 102. The MCT detector 102 converts a change in its electricalresistance caused by the infrared emission into a voltage and therebygenerates a detection signal.

[0102] A preamplifier amplifies the output from the MCT detector 102 by100 times, and a lock-in amplifier detects and amplifies the in-phasesignal. A digital sampling oscilloscope (not shown) triggered by anoutput from the optical chopper 209 displays the waveform of thedetection signal. The detection signal is input to an A/D conversionboard of a computer and subjected to data processing as follows. Adecoder decodes the processed signal.

[0103]FIG. 12 shows data of a differential coefficient obtained bysmoothing the detection signal of the infrared emission from the sampleand differentiating the smoothed signal as in Example 1. In thisexample, barcodes were read by using the JAN code rules. As describedearlier, a JAN code of eight characters is composed of left guard bars“101”, four data characters on the left side, center bars “01010”, threedata characters on the right side, one modular check character, andright guard bars “101”. One character (number) is expressed by forming amodule seven times as large as the basic width by using two bars and twospaces.

[0104] In the differential coefficient data shown in FIG. 12, the leftguard bars “1101” are first detected. The center bars “01010” are thendetected by looking up the left guard bar detection signal. The leftguard bars are removed from the measured signal. Next, the differentialcoefficient shown in FIG. 12 is divided into each characters within leftto center bars by two bars and two spaces. Meanwhile, reference signalsof modules corresponding to individual numbers are obtained. Thecorrelation between each reference signal and an actually measuredsignal is calculated to obtain a number by which the correlated value ofthe two signals is a maximum. By this manipulation, numberscorresponding to the modules divided as above are determined.

[0105] In this example, when the upper surface of the sample was heatedto 75° C., the barcode read accuracy was 90% or more. However, the readaccuracy was about 50% and about 90% when the upper surface of thesample was 50° C. and 90° C., respectively.

[0106] Instead of the sample (A), samples (B) and (C) were prepared byforming barcodes as follows.

[0107] Sample (B): A polyacrylonitrile powder was dispersed in a 5 wt %aqueous polyvinyl alcohol solution at a ratio of 2 wt % with respect topolyvinyl alcohol. The resultant material was used to form a barcode onplain paper.

[0108] Sample (C): An acrylonitrile-styrene copolymer (AS resin) wasdissolved in toluene, and the solution was mixed with wax. A filmsubstrate was coated with the resultant material and dried to form aheat-sensitive ink ribbon. This heat-sensitive ink ribbon was used toform a barcode on plain paper.

[0109] Results similar to those described above were obtained when thesesamples (B) and (C) were used.

Example 7

[0110] As shown in FIG. 13, a thermal head 216 was used as heating meansinstead of the tubular halogen lamp in Example 6. This thermal head 216has a mechanism for adjusting its vertical position. When a sample movesto the heating position, the thermal head 216 is moved downward andpushed against the sample. The pulse width of a pulse voltage to beapplied to the thermal head 216 is related to the temperature on theupper surface of the sample and the signal intensity of infraredemission. In this manner a pulse voltage with an appropriate pulse widthis applied to the thermal head 216.

[0111] As in Example 6, a pulse stage 202 is set in the detectionposition. After a sample 1 is placed on a stage top plate 203, an airvalve is activated to move the sample 1 together with the stage top:plate 203 to the heating position. The thermal head 216 heats the uppersurface of the sample 1 to 75° C. in the initial position. After that,the stage top plate 203 is immediately returned to the initial position,and infrared emission is detected in the process of cooling as inExample 6.

[0112] The same sample (A) as used in Example 6 was used to detectinfrared emission from a barcode following the same procedure as inExample 6. Data optimized in the same manner as in Example 6 was decodedby a decoder. Consequently, the decoded data matched the result ofdecoding of the original data printed on the existing article.

Example 8

[0113] When measurement is performed in the same manner as in Example 7,a signal of a peripheral portion is detected as follows in addition to asignal of a barcode information portion shown in FIG. 14. This signal ofthe peripheral portion is used as a background level to correct thedetection signal of the barcode information portion.

[0114] A pulse stage 202 is set in the detection position, and a sample1 is placed on a stage top plate 203. After the barcode informationportion is aligned in this initial position, an air valve is activatedto move the sample 1 together with the stage top plate 203 to theheating position. A thermal head 216 heats the upper surface of thesample 1 to 75° C. in the initial position. After that, the stage topplate 203 is immediately returned to the initial position. In thesubsequent cooling process, while the pulse stage 202 is moved at afixed rate of 20 mm/sec, infrared emission from the barcode informationportion is detected and input as a file to a computer.

[0115] Next, the stage top plate 203 is returned to the initialposition, and the optical axis of the detecting optical system isaligned with the peripheral portion of the barcode information portion.After that, infrared emission from the peripheral portion is detectedand input as a file to the computer.

[0116] As data processing, smoothing, binarization by differentiation,and width adjustment were performed following the same procedures as inExample 1 for a difference obtained by subtracting the peripheralportion signal from the barcode information portion signal. Dataoptimized in the same manner as in Example 1 was decoded by a decoder,and the decoded data matched the result of decoding of the original dataprinted on the existing article.

Example 9

[0117] When measurement is performed in the same manner as in Example 7,a signal of a peripheral portion is detected as follows in addition to asignal of a barcode information portion shown in FIG. 14. This signal ofthe peripheral portion is used as a background level to correct thedetection signal of the barcode information portion.

[0118] A pulse stage 202 is set in the detection position, and a sample1 is placed on a stage top plate 203. After the barcode informationportion is aligned in this initial position, an air valve is activatedto move the sample 1 together with the stage top plate 203 to theheating position. A thermal head 216 heats the upper surface of thesample 1 to 75° C. in the initial position. After that, the stage topplate 203 is immediately returned to the initial position. In thesubsequent cooling process, the pulse stage 202 is scanned in adirection S at a fixed rate of 20 mm/sec. Synchronizing with thisscanning, rollers 201 are rotated to reciprocate the sample 1 in adirection L at a rate of 200 mm/sec such that the barcode informationportion and the peripheral portion are alternately scanned across theedge (the boundary between the information portion and the peripheralportion) of the barcode information portion shown in FIG. 14. In thismanner, infrared emission from the barcode information portion and thatfrom the peripheral portion are detected. The obtained signals areamplified by an AC coupled-amplifier and input as a file to a computer.

[0119] Smoothing, binarization by differentiation, and width adjustmentwere performed for the obtained data. When the optimized data wasdecoded by a decoder, the decoded data matched the result of decoding ofthe original data printed on the existing article.

Example 10

[0120]FIG. 15 is a block diagram showing a barcode reading apparatusused in this example. This apparatus uses a silicon macro camera lens asan optical element for condensing and guiding infrared emission from asample to a detector, and a PtSi FPA-CCD as an infrared detector.

[0121] As in the apparatus shown in FIG. 9, a pulse stage and a stagetop plate 203 are mounted on rollers. A sample 1 is placed on the stagetop plate 203 and aligned by looking up a target 204. When the sample 1moves to the heating position, a tubular halogen lamp 206 with reflectorheats the sample 1. In the detection position, a silicon macro cameralens 301, a bandpass infrared filter 302, and a PtSi FPA-CCD 303 arepositioned above the sample 1. The silicon macro camera lens 301includes an antireflection coating and has a viewing angle of 20°×15°and a minimum focal length of 250 mm. The FPA-CCD 303 has ahighest-sensitivity wavelength of 4.5 μm, includes 320×240 pixels, andis cooled to −200° C. by a Stirling cooler 304 when in use. Electriccharge stored in the EPA-CCD 303 is transferred to a reader 305synchronizing with a sync signal. The signal transferred to the reader305 is amplified by a preamplifier 306, input to an A/D conversion board308 of a computer 307, and decoded by a decoder 309.

[0122] The pulse stage is set in the detection position, and the sample1 is placed on the stage top plate 203. The center of the FPA-CCD 303and the center of a barcode information portion of the sample 1 arealigned with reference to the target 204. An air valve is activated tomove the sample 1 together with the stage top plate 203 to the heatingposition. The halogen lamp 206 heats the upper surface of the sample 1to 75° C. in the initial position. After heating, the stage top plate203 is immediately returned to the initial position, and infraredemission is detected in the cooling process.

[0123] The infrared emission from the sample 1 was input as atwo-dimensional image file to the computer 307. Smoothing and signalintegration within a set range were performed for this two-dimensionalmeasured data to obtain one-dimensional data. This one-dimensional datawas subjected to binarization by differentiation and width adjustment.The processed data was decoded by the decoder 309, and the decoded datamatched the result of decoding of the original data printed on theexisting article.

Example 11

[0124] Toner made from the same AS resin as used in Example 6 was usedas toner of a laser beam printer to form an invisible barcode on plainpaper. This barcode corresponds to a standard-size barcode with a basicwidth of 300 μm printed on an existing article selected at random.

[0125] When the barcode is read with spatial resolution higher than thebasic width of its module, a signal having a time width corresponding tothe width of a bar or a space is obtained (FIG. 16A). When the barcodeis read with spatial resolution equivalent to the basic width of itsmodule, the read time can be shortened, but the apparatus function issuperposed (convoluted) on the amplitude of a signal (FIG. 16B).Consequently, as can be seen by the comparison of the signal shown inFIG. 16B with the signal shown in FIG. 16A, peaks of the signal with lowintensity become low, and this obscures the correspondence between thetime width of the signal and the width of a bar or a space. Therefore,it is desirable to allow a filter that serves as a function of thefrequency of a detection signal as shown in FIG. 16B to act on thesignal to remove (deconvolute) the apparatus function and correct theamplitude and then binarize the signal.

[0126] In this example, an invisible symbol reading apparatus as shownin FIG. 6 was used, the temperature was controlled by turning on and offa temperature controller, and the barcode was read in the coolingprocess as follows. A sample 1 was held on a holder 32 by a plate-likemagnet and so adjusted that a barcode information portion of the sample1 was positioned. at the focal point of a detecting optical system bylooking up a target 34. To adjust the optical axis, guide light with awavelength of 640 nm was irradiated from a semiconductor laser (notshown). However, the focal point of this guide light shifted from thefocal point of infrared emission. Hence, after the in-plane position ofthe optical axis was adjusted by the guide light, final adjustment wasperformed by monitoring an output from an MCT detector 17 on anoscilloscope. The temperature to which the sample 1 was heated under thecontrol of a temperature controller 33 was adjusted to 90° C. A signalfrom a built-in thermocouple (not shown) of the holder 32 was input tothe temperature controller 33. On the basis of this signal, the currentto be supplied to a bar heater 31 was adjusted. Before the sample 1 wasscanned, the temperature controller 33 was turned off to put the sample1 in the cooling process. While a pulse stage 11 was moved at a fixedspeed by a signal from a stage controller 12, infrared emission at thefocal point was detected. The start and end positions of the barcodewere measured by taking margins for these positions by using limitswitch signals from the pulse stage 11. Data of the detection signal wasinput as a file to a computer 21.

[0127] If the signal intensity is high, an optical stop can be installedon the optical axis to reduce wavelength components except for thewavelength to be measured, thereby effectively performing wavelengthselection. If this is the case, a bandpass infrared filter 16 isunnecessary.

[0128] Filtering was performed for actually measured data as shown inFIG. 16B to remove the apparatus function and smooth the data. Afterthat, binarization by differentiation and width adjustment wereperformed in the same manner as in Example 1. When the data was decodedby a decoder, the decoded data matched the result of decoding of theoriginal data printed on the existing article.

Example 12

[0129] A heat-sensitive ink ribbon similar to that used in the formationof the sample (C) in Example 6 was used to form an invisible barcode ona plastic card. This barcode corresponds to a standard-size barcode witha basic width of 300 μm printed on an existing article selected atrandom.

[0130] In this example, a barcode reading apparatus having heating meansand a detecting optical system shown in FIG. 17 was used. This readingapparatus has a thermal bar 51 separated from a lens barrel 101 of thedetecting optical system including a Cassegrain lens and the like. Aguide 52 is formed below these members. A card type sample 1 is placedwith its barcode printed surface facing up on the guide 52 having acurved portion, and is conveyed by rotation of a roller 53. An opticalsensor 54 installed before the heating position senses the approach ofthe sample 1, and the temperature of the thermal bar 51 is raised by thesensor signal. The thermal bar 51 is usually operated by remaining heatin order to prevent thermal deterioration of the opposing roller 53. Thesample 1 is pushed and heated by the thermal bar 51 while passingthrough the gap between the curved portion of the guide 52 and thethermal bar 51. A thermocouple senses the temperature of the backsurface of the sample 1, and a temperature controller controls theheating temperature. The sample 1 whose barcode surface is heated by thecontact with the thermal bar 51 is conveyed toward the infrared emissionsensing position by the roller 53. An optical sensor 55 installed beforethe sensing position senses the passage of the sample 1 and supplies ameasurement start signal to a computer. Infrared emission is detected inthe cooling process. This infrared emission from the barcode on thesample 1 is guided into the lens barrel of the detecting optical system.The infrared emission intensity as a function of time (convertible intoa function of position because the roller rotates at a fixed rate) isinput as a file to the computer.

[0131] Following the same procedures as in Example 1, smoothing,binarization by differentiation, and width adjustment were performed forthe data. The data optimized in the same manner as in Example 1 wasdecoded by a decoder. Consequently, the decoded data matched the resultof decoding of the original data printed on the existing article.

[0132] In the apparatus shown in FIG. 17, the rotating speed of theroller 53 can be so changed as to increase the conveyance speed of thesample 1 when the thermal bar 51 heats the sample 1 and decrease theconveyance speed of the sample 1 when the optical system detectsinfrared emission.

[0133] Also, as in an apparatus shown in FIG. 18, a plurality of rollers61, 62, 63, 64 a, 64 b, 65, 66, 67 a, and 67 b can be used. In thisapparatus, the rotating speeds of these rollers are changed to increasethe conveyance speed of the sample 1 when the thermal bar 51 heats thesample 1 and decrease the conveyance speed of the sample 1 when theoptical system detects infrared emission.

[0134] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

1. A method for reading an invisible symbol comprising the steps of:heating an invisible symbol formed on a sample and containing a materialwhich emits infrared light when heated; detecting infrared light emittedfrom the invisible symbol; calculating a differential coefficient of adetection signal corresponding to a position on the sample; determining,on the basis of upper and lower threshold values set for thedifferential coefficient, a maximum value of the differentialcoefficient in a region exceeding the upper threshold value and aminimum value of the differential coefficient in a region smaller thanthe lower threshold value; and binarizing the detection signal by usingthe maximum or minimum value as a leading or trailing edge of a binaryfunction.
 2. The method according to claim 1, wherein the step ofdetecting infrared light emitted from the invisible symbol is performedin a process of cooling the sample.
 3. The method according to claim 1,wherein the invisible symbol is a linear barcode, a basic width of thelinear barcode is calculated from the binary function, and the binaryfunction is corrected to an integral multiple of the basic width.
 4. Themethod according to claim 1, wherein the invisible symbol is a linearbarcode, a basic width of the linear barcode is calculated by detectinga reference code pattern, and a data character is read on the basis ofthe calculated basic width.
 5. The method according to claim 1, whereina signal level of an underlying substrate is used as a signal level ofbackground to correct a signal level of the invisible symbol.
 6. Themethod according to claim 1, wherein the invisible symbol is made from apolymer containing a cyano group.
 7. The method according to claim 1,wherein the sample is heated to 60 to 100° C.
 8. An apparatus forreading an invisible symbol comprising: heating means for heating aninvisible symbol formed on a sample and containing a material whichemits infrared light when heated; detecting means for detecting infraredlight emitted from the invisible symbol; and an arithmetic operationunit for binarizing a detection signal from said detecting means.
 9. Theapparatus according to claim 8, wherein said arithmetic operation unitcalculates a differential coefficient of the detection signalcorresponding to a position on the sample, determines, on the basis ofupper and lower threshold values set for the differential coefficient, amaximum value of the differential coefficient in a region exceeding theupper threshold value and a minimum value of the differentialcoefficient in a region smaller than the lower threshold value, andbinarizes the detection signal by using the maximum or minimum value asa leading or trailing edge of a binary function.
 10. The apparatusaccording to claim 8, wherein said seating means for heating the sampleis installed in a position apart from said detecting means, and saidapparatus further comprises means for moving the sample from a heatingposition of said heating means to a detection position of said detectingmeans.
 11. The apparatus according to claim 8, further comprisingcontrol means for turning off said heating means heating the samplebefore detection by said detecting means.
 12. The apparatus according toclaim 8, further comprising: means for optically modulating the infraredlight emitted from the invisible symbol; and means for detecting a phaseof the detection signal.
 13. The apparatus according to claim 8, furthercomprising a bandpass infrared filter for transmitting infrared light ina specific wavelength region of the infrared light emitted from theinvisible symbol.
 14. The apparatus according to claim 13, wherein saidbandpass infrared filter transmits infrared light near 4.5 μm peculiarto a cyano group.
 15. The apparatus according to claim 8, furthercomprising a calcium fluoride lens as condensing means.
 16. Theapparatus according to claim 8, further comprising a Cassegrain lens ascondensing means.
 17. The apparatus according to claim 8, wherein saiddetector is a mercury cadmium tellurium detector.
 18. The apparatusaccording to claim 8, wherein said detector forms a focal plane array.19. The apparatus according to claim 18, wherein an element constructingsaid focal plane array is made of a material selected from the groupconsisting of platinum silicide and indium antimony.